It is well known that many substances are prone to crystallize in different manners, depending on the conditions under which they are crystallized. Different crystalline structures resulting from crystallization of a particular substance are called polymorphs or pseudopolymorphs. It is also known that, when they are melted and cooled rapidly below their melting point, i.e. melt-congealed, the atoms or molecules forming most substances need some time to arrange themselves in the order most natural for the environment in which they are placed. Accordingly, they remain in unstable amorphous or semiamorphous states or organize into metastable polymorphs.
Metastable polymorphs may be enantiotropic, which is a property of certain substances meaning that they can exist in more than one crystal form (Giron, Thermal Analysis and Calorimetric Method in the Characterization of Polymorphs and Solvates, Thermochimica Acta, 248 (1995) 1-59; Parker, Dictionary of Scientific and Technical Terms, McGraw Hill, Inc., 1984, 541; Hancock et al., Characteristics and Significance of the Amorphous State in Pharmaceutical Systems, J. Pharm. Sci., Vol 86, No. 1, 1997, 1-12). Often, there is a relation between the various crystal forms or habit of an enantiotropic substance such that one form is stable above the transition-point temperature and the other is stable below it. Consequently, the crystal habit is dynamic and reversible depending on ambient conditions.
Metastable polymorphs often transform over time into more stable structures. This natural crystallization process is called xe2x80x9cagingxe2x80x9d, and occurs over time without human intervention. This natural xe2x80x9cagingxe2x80x9d process is often lengthy and unpredictable, and therefore is costly and potentially dangerous, particularly in the manufacture of pharmaceuticals. The unpredictability arises since the aging process largely depends on environmental factors. Yu, xe2x80x9cInferring Thermodynamic Stability Relationship of Polymorphs from Melting Dataxe2x80x9d, J. Pharm. Sci., Vol 84, No. 8, 966-974 (1995).
Nevertheless, stable, crystallized substances are generally required for optimum and reliable bioactivity and bioavailability. If metastable particles, for example, microspheres or pellets, are placed in an aqueous medium before full crystallization occurs, deformation of particle shape or even complete destruction of the particles can occur in a matter of hours.
Furthermore, different polymorphs of a particular substance will have different dissolution rates, resulting in a lack of stability and loss of uniformity between different batches of the same drug. For example, Haleblian et al report differences in dissolution rates between polymorphs of fluprednisolone. Haleblian et al. xe2x80x9cIsolation and Characterization of Some Solid Phases of Fluorprednisolonexe2x80x9d, J. Pharm. Sci., Vol. 60, No. 10, 1485-1488 (1971).
For pharmaceutical applications, it is particularly important to achieve stable crystallization, because administration of a therapeutic compound often requires suspension in an aqueous solution suitable for injection. Also, even if the compound is not first suspended in an aqueous medium, when it is administered to the patient it is subjected to biological fluids that are water based. The same is true for pellets and implants that are placed in the body through a surgical or other procedure. To assure the physical integrity of the shaped particles and uniform release of the active agent, it is necessary to assure full crystallization prior to administration.
Some workers have attempted to improve the stability of therapeutic compounds by inducing crystallization. For instance, Matsuda et al. suggest modifying crystalline structures using a temperature controlled dispersion drying method. Matsuda et al. xe2x80x9cPhysicochemical Characterization of Sprayed-Dried Phenylbutazone Polymorphsxe2x80x9d, J. Pharm. Sci., Vol 73, No. 2, 73-179 (1984).
However, because dissolution of a solid is also related to surface erosion, the shape and size of the therapeutic particles must also be considered in addition to solubility. Carstensen, xe2x80x9cPharmaceutical Principles of Solids and Solid Dosage Formsxe2x80x9d, Wiley Interscience, 63-65, (1977). Thus, when a pharmaceutical compound is administered as a solid or suspension, the preservation of particular shape and size becomes an important factor for assuring the control and reproducibility of bioavailability and biodynamics.
With this in mind, Kawashima et al. proposed a method of spherical crystallization of Tranilast through the use of two mutually insoluble solvents, and conversion of the resulting polymorphs by means of heat. Rawashima et al., xe2x80x9cCharacterization of Polymorphs of Tranilast Anhydrate and Tranilast Monohydrate When Crystallized by Two Solvent Change Spherical Crystallization Techniquesxe2x80x9d in J. Pharm. Sci., Vol 80, No. 5, 472-477 (1981).
It has also been reported that the natural process of aging can be accelerated through heating. Ibrahim et al., xe2x80x9cPolymorphism of Phenylbutazone: Properties and Compressional Behavior of Crystalsxe2x80x9d in J. Pharm. Sci., Vol 66, No. 5, 669-673 (1977); Hancock et al., Characteristics and Significance of the Amorphous State in Pharmaceutical Systems, J. Pharm. Sci., Vol 86, No. 1, 1-12 (1997). In some cases, however, the heat required is such that the integrity or shape of the substance is compromised. In several cases where heat has been used, reproducibility of results, stability, and hence control of crystal size within particles has been difficult or even impossible to achieve.
In addition, in some cases the most stable polymorph of a particular substance is a hydrate, rendering it impossible to reach the desired polymorph by means of heat due to resulting dehydration. Furthermore, heating is rarely appropriate for stable crystallization in the case of mixtures. Thus, the process of heat as a method for obtaining stable polymorphs, though superior to the aging process, has significant limitations.
Other workers have studied the use of solvent vapors to induce crystallization of polymeric species. Such efforts include putative crystallization and change of the mechanical properties of polymeric compounds, as described in U.S. Pat. No. 4,897,307. See also M{overscore (u)}ller, A. J. et al., xe2x80x9cMelting behavior, mechanical properties and fracture of crystallized polycarbonatesxe2x80x9d in Latinoamericana de Metalurgia y Materiales, 5 (2), 130-141 (1985); and Tang, F. et al., xe2x80x9cEffect of Solvent Vapor on Optical Properties of Pr/sub 4 VOPe in polymethylmethacrylatesxe2x80x9d, in Journal of Applied Physics, 78 (10), 5884-7 (1995).
Tang et al. used organic solvent vapors to transform a polymer matrix, Pr4VOPc dye (Vanadyl Phtalocyanine having 4 propyl substituents) from glassy phase I to crystallized phase II. M{overscore (u)}ller and Paredes describe the crystallization of polycarbonate polymers in terms of the incorporation of solvents or plasticizers into the amorphous state. To the knowledge of the present inventors, such an approach has not been used to form stable crystals of melt-congealed organic compounds and mixtures.
The present invention provides reproducible, stable particles of crystalline organic compounds. The stable particles of crystalline organic compounds of the present invention might be homogeneous particles of a singular organic compound, or they might be mixtures of two or more organic compounds. The stable particles of the present invention retain a constant shape and size during prolonged storage, such as in an aqueous suspension. Such stable particles can be fabricated to a uniform size and shape, and will retain said size and shape despite long term storage; and thus, are particularly advantageous in pharmaceutical formulations. The present invention further provides a method for obtaining such reproducible, stable particles. The method involves exposing the above shaped particles, wherein the one or more organic compounds is in a crystalline, amorphous, or some metastable form, to an atmosphere saturated with solvent vapors. The solvents are comprised of one or more liquids in which at least one or more of the organic compounds is soluble.
The method of the present invention affords several advantages. It is applicable to substances where the most stable polymorph is a hydrate, because it does not drive off water molecules and thereby allows the incorporation of water molecules into the crystalline web during formation. It is applicable to thermolabile substances, since high temperatures are avoided. And it allows stable structure formation involving a mixture of substances, which, with the exception of the eutectic mixture composition, can not be attained by means of heat.
More particularly, the present invention involves a method of crystallizing or recrystallizing an amorphous or metastable crystalline organic compound or mixture. The method comprises the steps of (i) exposing said compound or mixture to an atmosphere saturated with the vapors of one or more liquids, at least one of which must be a solvent for said compound or mixture, for a time sufficient for transforming the metastable compound or mixture to a stable, crystallized compound or mixture; and (ii) recovering the stable, crystallized compound or mixture for storage or use.
The method may be performed using any enclosure where the volume, temperature, and atmospheric content and pressure can be manipulated. The chamber is capable of containing an atmosphere saturated with the desired solvent vapors. The point of saturation is reached when the vapors fill the chamber without causing condensation on the surfaces of the chamber or the particles.
Preferably the particles are formed into a shaped particle, such as a microsphere, pellet or implant form. Particles configured to have uniform and reproducible surface area are especially preferred. This can be effected by melt-congealing. Further, the shaped particles are preferably configured into a uniform particle size or range of sizes. To this end, the methods described in U.S. Pat. Nos. 5,633,014, 5,643,604, and 5,360,616 can be used, which are herein incorporated by reference. Alternatively, any suitable method that results in a metastable crystalline conglomeration can be used. Where the method involves crystallization of a mixture, the mixture may be eutectic or noneutectic.
The particles are placed in the chamber or other suitable enclosure using any suitable means such that they are exposed to solvent vapors, but not immersed in or otherwise contacting liquid solvent. The particles are stationary or mobilized within the chamber.
The time period necessary for effecting crystallization in accordance with the present method will vary depending on various physicochemical properties consistent with established principles. For example, the optimal time of exposure will vary depending on the shape and size of the particle, the chemical makeup of the particle, the form of the solid state of the particle (i.e., amorphous, metastable crystalline), the type and concentration of solvent used, and the temperature of the treatment. Generally, a range of several seconds to 48 hours is applied, or more preferably, 1 to 36 hours. Previous partial crystallization of particles does not appear to modify these time ranges. Optimization of the time of exposure will vary depending on the solvent system used, the organic compound(s) to be crystallized, and other variables, and is within the skill of one of ordinary skill in the art. As shown below, a 24 hour exposure time will commonly be effective.
One advantage of the present invention is that it is applicable to thermolabile substances because high temperatures may be avoided. Thus, the applicable temperature range is broadly defined and dependent on the particular compound. Generally, the temperature of the vapor atmosphere is sufficient to obtain vaporization of the solvent, but below the melting point of the particles.
The solvent or solvents used in the method of the present invention can be any agent classified as a solvent for the organic compound(s) of interest. As will be appreciated by any ordinarily skilled worker in the art, the selection of solvent will depend on the compound(s) sought to be stabilized. Exemplary solvents are conventional laboratory liquid solvents such as water, alkanes, alkenes, alcohols, ketones, aldehydes, ethers, esters, various acids including mineral acids, carboxylic acids and the like, bases, and mixtures thereof. Some specific exemplary solvents are methanol, ethanol, propanol, acetone, acetic acid, hydrochloric acid, tetrahydrofuran, ether and mixed ethers, pentane, hexane, heptane, octane, toluene, xylene, and benzene. Water is an especially useful component of a solvent/liquid mixture of the present invention, particularly where the most stable polymorph of a substance is a hydrate. Generally, solvents suitable for conventional liquid recrystallization of the compound of interest are suitable as a solvent in the present method.
The compound(s) of the stable particles of the present invention include any organic compound capable of existing as a crystalline solid at standard temperature and pressure. A preferred embodiment of the present invention is that wherein the particles are comprised of one or more organic compound(s) capable of forming into a stable crystalline solid. Preferably, the stable crystalline solid is a lattice of discrete organic molecules, i.e., non-polymeric.
Also preferred are organic compounds having some pharmacological or therapeutic activity. Still more preferred are such pharmacological compounds susceptible to the formation of polymorphs. Preferred embodiments further include particles comprised of a steroid or sterol, such as estrogen, 17xcex2-estradiol, testerone, progesterone, cholesterol, or mixtures thereof. These mixtures can also include Oxatomide/Cholesterol, Niphedipine/Cholesterol, Astemizol/Cholesterol, which have non-steroidal components. Stable shaped particles of other organic compounds are also provided by the present invention, e.g., Cisapride, Oxatomide.
Because the method of the present invention results in significant stabilization of particles of amorphous or metastable crystalline organic compounds, the particles of the present invention can be stored in liquid suspension, such as aqueous medium, or administered directly to a patient. Because the present invention provides stable forms of existing pharmacological agents, it will be understood by those skilled in the art that the particles of the present invention can be used in accordance with conventional practice in analogous formulations, e.g., the parenteral administration of microspheres, administration of pharmacological agents via implants, etc.
All publications and patent applications referenced herein are incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described.
The present invention provides stable shaped particles of one or more allotropic molecular organic compounds. Allotropic organic compounds are those capable of assuming two or more distinct physical forms (e.g., assuming different crystalline forms or an amorphous versus a crystalline form). Such allotropic species are also referred to as polymorphs or polymorphic species.
The storage stable, shaped particles of the present invention optionally further comprise pharmaceutically acceptable excipients, stabilizers, and buffers as are commonly known among those in the pharmaceutical arts.
These stable shaped particles possess an advantageous combination of physicochemical properties. First, the particles are configured into desired shapes by means that might not result in the most stable crystalline form of the constituent organic compound. The particles are then subjected to a solid state crystallization process that results in the organic compound assuming the most stable crystalline structure, and facilitates the retention of the size and shape of the original particle. The resultant product is a particularly configured particle comprised of one or more molecular organic compounds, each having a uniform crystalline character and possessed of a high degree of storage stability.
The combination of the uniformity of size and shape of the particle and the uniformity and stability of the crystalline structure of the constituent organic compound lends particular predictability and consistent bioavailability and associated biodynamics.
More particularly, the particles are pre-fabricated to desired specifications, e.g., microspheres of particular size and shape. The particles are then subjected to a solid state crystallization process that stabilizes the compounds of the particles without loss of the pre-fabricated size and shape. The resulting particles have greater uniformity of size and shape, more uniform and predictable dissolution profiles, and greater storage stability in various forms, e.g., in liquid suspension such as aqueous media or other storage liquid, as lyophilized solid, or alone as a powder or dry solid. By storage stable is meant the particles have improved shelf life without loss of the desired uniform size and shape of the particles, per se. That is, if the desired particle shape is a microsphere, the particles will retain a spherical shape of constant size over periods exceeding several years.
As used herein, storage stable refers to retention of the original size and shape of the particle, as well as the pharmacological activity of the active agent over a period of at least one month.
The present invention also involves a method of crystallizing shaped particles of a metastable compound or mixture of compounds without dissolution of the particle and attendant loss of the desired shape. The crystallization process is effected by exposing said particles to a controlled atmosphere saturated with the vapors of a solvent or solvents. The atmosphere is optionally modified in other respects, e.g., pressure, temperature, inert gases, etc. Preferably, the controlled atmosphere is saturated with a solvent vapor but not so much as to effect condensation of said solvent.
More particularly, the method of the present invention involves effecting crystallization of an amorphous or metastable organic compound in a shaped particle without alteration of the dimensions (e.g., size and shape) of said particle comprising: (i) exposing said shaped particle to an atmosphere saturated with the vapor of a liquid, said liquid being a solvent for said organic compound; and (ii) recovering said shaped particle wherein said organic compound is of a uniform crystalline structure.
Alternatively stated, the method involves effecting a solid state crystallization of a molecular organic compound in a particle of definite size and shape comprising: (i) exposing said particle to an atmosphere saturated with a solvent for said organic compound; and (ii) recovering said particle, wherein said organic compound in said recovered particle is of a uniform crystalline structure, and said recovered particle has retained said size and shape. Retaining the size and shape of the particle is meant to include minor variations in the dimensions of the particle, e.g., no more than about 15%; and preferably, no more than about 10%.
The present invention provides a means for fabricating particles of desired dimension without regard to the resulting allotropic form of the organic compound. After the particle is fabricated into the desired shape and size, the solid state crystallization can be effected to crystallize the organic compound into a storage stable solid state of uniform crystal structure. Moreover, the solid state crystallization of the present invention can be effected on particles comprised of more than one allotropic organic compound.
Preferably, the shaped particle is a microsphere; and, as a result of the present process, the organic compound(s) of the microsphere are ordered into a single, homogeneous crystalline form without any deterioration in the size or shape of the microsphere.
For purposes of the present invention, the term xe2x80x9ccrystallizationxe2x80x9d refers to a process by which the most stable polymorph of a particular substance is achieved. Recrystallization refers to a process similar to crystallization except that the organic compound of the particle, rather than being amorphous, was initially only partially crystalline, of a mixed crystalline habit, or crystalline, but of a less stable crystalline form. Unless indicated otherwise, the term crystallization includes recrystallization.
The term xe2x80x9csolid state crystallizationxe2x80x9d refers to a crystallization process that is effected without macroscopic dissolution of the compound being crystallized. As used herein, solid state crystallization includes a crystallization process wherein an organic compound within a shaped particle is crystallized or rescrystallized by exposure to a solvent vapor without loss or alteration of the shape or size of the particle. It will be appreciated by those skilled in the art that while subtle intermolecular changes will be effected by such crystallization (e.g., creation or rearrangement of crystal lattice structure), the microscopic and/or macroscopic dimensions of the particle will not be appreciably altered.
The term xe2x80x9csaturatedxe2x80x9d when used in reference to the atmosphere wherein the crystallization is conducted means that the atmosphere within the chamber or enclosure used to hold the solvent vapors contains the maximum quantity of said solvent in the vapor phase without effecting visible condensation on surfaces within the chamber. Condensation does not include microscopic condensation on the surface of the particles that does not affect their shape.
The term xe2x80x9csolventxe2x80x9d refers to a liquid at standard temperature and pressure, and one capable of solubilizing an appreciable amount of a specified solid solute. The solid solute will be a particular organic compound. Solids vary from 0-100% in their degree of solubility. See, e.g., xe2x80x9csolubility Parameters of organic Compoundsxe2x80x9d, CRC Handbook of Chemistry and Physics, 62d ed., C-699, CRC Press; N. Irving Sax and Richard J. Lewis, Sr., Hawley""s Condensed Chemical Dictionary, 11th ed., 1079 (1987). For purposes of the present invention, a liquid will be a considered a solvent with respect to a particular solid solute provided the solute is at least 10% soluble in said liquid.
The term xe2x80x9cparticlexe2x80x9d refers to a discrete collection of a plurality of molecules of one or more organic compounds. As used herein, a particle may be an ordered collection (e.g., crystalline) or disordered collection (e.g., amorphous) of molecules, or any combination thereof. The term embraces, among other things, microscopic as well as macroscopic particles such as powders, microspheres, pellets, implants, and the like.
Preferably, particles are made of microspheres. The preferred microspheres of the present invention range in size from 1 micron to 1 millimeter, more preferably 1 to 500 microns, and most preferably in the range of 1 to 100 microns, particularly for human use. When the particles are in pellet form, such particles are normally but not necessarily cylindrical with lengths of 1000 to 5000 microns and diameter of 500 to 1000 microns. These particles can have important applications for veterinary use, and are not injected but deposited under the skin.
The size and shape of the particle will depend on the intended application and the constituent organic compound(s). For example, microsphere size is chosen for practical reasons, i.e. a size appropriate for administration using a hypodermic needle or for assuring a desired rate of dissolution.
The term xe2x80x9cmolecular organic compoundxe2x80x9d refers to an organic compound existing as stable discrete molecules (i.e., non-polymeric) and when combined with a plurality of identical molecules is capable of assuming one or more ordered crystalline structures. Thus, a molecular organic compound is meant to distinguish from a polymeric species.
The term xe2x80x9cmetastablexe2x80x9d means a pseudoequilibrium state of a solid substance where the content of free energy is higher than that contained in the equilibrium state. For our particular purposes, a xe2x80x9cstablexe2x80x9d substance or particle has a crystalline structure whose shape remains unchanged in a standard ambient environment, e.g. in air having varying levels of moisture, for an extended period of time. However, it should be understood that xe2x80x9cstablexe2x80x9d does not indicate infinite stability, but means sufficiently stable such that the particles remain sufficiently stable for the preservation of their crystalline characteristics during storage and up to their application and use and additionally, after they have been administered to a subject, up to their total dissolution.
The present invention also encompasses stable microspheres achieved using the present method. Such microspheres preferably contain a compound having pharmaceutical applications. The microspheres and pellets of the present invention are useful in human, as well as animal, therapeutic regimens.
For instance, there is currently a need for compositions that accomplish the sustained release of steroid growth promoters in food animals to promote the growth of such animals. The amount of growth hormone administered to an animal would depend on the particular animal species, hormone, length of treatment, age of animal, and amount of growth promotion desired. Other considerations to be made in the use of hormonal compositions in the treatment of animals are discussed in U.S. Pat. No. 5,643,595, which is herein incorporated by reference. The particles of the present invention can be particularly configured for optimal delivery by injection by varying the particle size.
As discussed above, the microspheres of the present invention are stable in aqueous fluids, and are thus amenable to parenteral injection. Modes of administration include but are not limited to intra-venous (IV), intra-arterial (IA), intra-muscular (IM), intra-dermal, subcutaneous, intra-articular, cerebro-spinal, epi-dural, intra-peritoneal, etc. In addition, the compounds of the present invention can be administered via an oral route, either as an aqueous suspension or a lyophilized product. Other routes of administration are also acceptable, including topical application, into the eye, or via inhalation in the form of droplets or mist.
The dosage form according to the present invention may take the form of a microsphere powder in vials/ampoules, ready to be prepared as suspensions, or take the form of ready-prepared suspensions, packaged into injectable ampoules or directly into syringes, ready to be administered in human or veterinary medicine. The suspension medium may be water, a saline solution, an oil, containing buffers, surfactants, preservatives, commonly used by pharmacotechnicians for preparing injectable substances or any other substance or combination which does not threaten the physical and chemical integrity of the substances in suspension and which is suitable for the organism which will receive it. If it is desired to avoid a sudden initial increase in the level of active ingredient in the internal medium of the receiving organism, it will be preferable in the case of ready-for-use suspensions to use liquid vectors in which said active ingredients are practically insoluble. In the case of active substances partially soluble in the lukewarm liquid vector but insoluble at cold temperature, it is preferable, from the pharmacological point of view, to avoid the formation of precipitates (called xe2x80x9ccakingxe2x80x9d effect) by preparing formulations in the form of separate microsphere powder and liquid vector which will be mixed only at the time of injection.
In veterinary applications, where the duration of the desired effect may be very long (for example the lactation period of the adult female), diameters of a few hundreds of microns may be used. If it is desired to limit the diameter of injection syringe needles for the comfort of the patient, the diameter of the microspheres should be limited to 300 microns and more preferably to 100 microns. In contrast, for very short durations of effect (for example circadians), the diameter of a microsphere may be reduced to 5 microns.
For most applications in human medicine (duration of action of the active ingredient between a circadian cycle and a menstrual cycle), it is preferable to use microspheres whose diameter is between 5 and 100 microns, depending on the combinations of active substances/carrier substances.
A separation of microspheres according to their diameter may be performed during the manufacturing process using known processes: for example, by cyclonic separators, by sieving using air suction or by sieving in aqueous medium. In practice, it is sufficient if more than 70% of the microspheres have diameters of between 70% and 130% of a specified diameter. If necessary, the ideal dissolution curve, determined by the proposed application, may be approached by mixing batches with suitable different diameters. Moreover, particles which do not comply with the specifications may be recycled.
The mechanism by which substances in a solid state crystallize in the presence of vapors containing at least one solvent has not yet been established. The crystallization process may well conform, as regards the effect of the solvents, to the traditional principles that apply in saturated solutions and in molecular mobility. It is possible that some molecular rotational or transference movement occurs, which seems to depend on the particular type of solvent used and to the temperature of vaporization. Hancock et al., xe2x80x9cCharacteristics and Significance of the Amorphous State in Pharmaceutical Systemsxe2x80x9d, J. Pharm. Sci., Vol 86, No. 1, 1-12(1997). It is clear however that the temperatures at which the crystallization is obtained are well below vitreous transition temperatures and are in fact only in accordance with that required for the solvents"" vapor pressure.
Without wishing to be bound by any theory, we contemplate that the vapor molecules of the solvent or solvents might form microcondensations and minute accumulations of solvent on the surface of the particles to be crystallized, thus bringing sufficient energy for the surface molecules of the solid particles to form organized structures (e.g., crystalline domains).
By the same token, if present in the vapor, water molecules become available for the formation of hydrates, when required for stable polymorphs.
Once the organizational and/or water absorbing process starts at the surface, it is possible that the crystallization process gradually spreads into the interior of the particle without the need for contact with or dissolution within the solvent.
If this is correct there are two facts which seem to indicate that these microcondensations or molecular agglomerations are extremely minute. First, if enough solvent condensation occurred on the surface of the particle, the solvent would at least partially dissolve it and modify its shape. To avoid any partial dissolution, the amounts deposited by the vapor must be extremely minute.
Second, during exposure to solvent vapors the particles, because of their small size and large quantity, inevitably come to contact with one another. Were there to be any surface dissolution of the particles, as would occur if the substantial quantities of amounts of deposited vapor were not very minute, the particles would tend to stick to each other and form lumps or agglomerates. Under the conditions described herein, this does not occur.